#include<iostream>
#include<vector>
#include<queue>
using namespace std;

class Solution {
public:
    int maxDistance(vector<vector<int>>& grid) {
        int m = grid.size(), n = grid[0].size();
        vector<vector<int>> dist(m, vector<int>(n, -1));

        queue<pair<int, int>> q;
        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                if (grid[i][j])
                {
                    dist[i][j] = 0;
                    q.push({ i,j });
                }
            }
        }
        int ret = -1;

        int dx[4] = { 0,0,1,-1 };
        int dy[4] = { 1,-1,0,0 };

        while (q.size())
        {
            int a = q.front().first;
            int b = q.front().second;
            q.pop();
            for (int k = 0; k < 4; k++)
            {
                int x = a + dx[k], y = b + dy[k];
                if (x >= 0 && x < m && y >= 0 && y < n && dist[x][y] == -1)
                {

                    q.push({ x,y });
                    dist[x][y] = dist[a][b] + 1;
                    ret = max(ret, dist[x][y]);
                }
            }
        }
        return ret;

    }
};